Walter Siebert

Universität Heidelberg, Heidelburg, Baden-Württemberg, Germany

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Publications (233)816.75 Total impact

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    ABSTRACT: Reaction of dilithio-o-carborane Li2C2B10H10 (generated in situ from o-carborane 1 and butyllithium) with the 1, 2-diborylbenzene derivative 1, 2-C6H4(iPr2NBCl)2 yielded the o-carborane compound 3 having the fused exo-polyhedral C2B2C2 heterocycle. The analogous reactions with 1, 1-bis(dimethylaminochloroboryl)ethane and with 1, 3-dichloro-1, 2,3-tris(dimethylamino)triborane(5) afforded the o-carborane compounds 5 and 7 containing the fused exo-polyhedral five-membered C2B2C and C2B3 rings, respectively. Attempts to use 1, 2-dichloro-1, 2-bis(dimethylamino)diborane(4) for the synthesis of the o-carborane compound 10 with a fused exo-polyhedral C2B2 ring instead led to the diborane(4)yl-o-carborane species 9c and 9d (the latter in trace amount), which contain a diborane(4)yl moiety bonded to the o-carborane. The molecular compositions of the new o-carborane derivatives follow from NMR spectroscopic and mass spectrometric data as well as from X-ray diffraction analyses of 3, 7 and 9c. The structures exhibit weak intramolecular C–H···H–B hydrogen-hydrogen interactions.
    Zeitschrift für anorganische Chemie 06/2013; 639(7). · 1.16 Impact Factor
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    ABSTRACT: Cationic triple-decker complexes with a bridging diborolyl ligand, [CpCo(μ-1,3-C3B2Me5)M(ring)]+ (M(ring) = CoCp (2a), CoCp* (2b), RhCp (3a), RhCp* (3b), IrCp (4a), IrCp* (4b), Ru(C6H6) (5a), Ru(p-MeC6H4Pri) (5b), Ru(C6Me6) (5c), Ru(η6-cycloheptatriene) (6)), were synthesized by reaction of CpCo(μ-1,3-C3B2Me5)Tl with [M(ring)Hal2]2. The structures of 2aBPh4, 2bPF6, 4aPF6, 5aOTf, and 5cPF6 were determined by X-ray diffraction. The electron-transfer ability of the complexes has been ascertained by electrochemical and spectroelectrochemical techniques. In general, they are able to shuttle reversibly in the sequence 2+/+/0/–, plausibly affording completely delocalized mixed-valence derivatives. DFT calculations revealed structural changes accompanying redox processes and satisfactorily predicted the potentials for the first reduction and first oxidation.
    Organometallics 04/2013; 32(9):2713–2724. · 4.15 Impact Factor
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    ABSTRACT: The reaction of CpCo(1,3-C3B2Me5H) (1) with CpTl affords the thallium derivative CpCo(1,3-C3B2Me5)Tl (2). The structures of 1 and 2 were determined by X-ray diffraction at 100 K. An “extra” hydrogen atom in 1 occupies a C–H···B bridging position. According to DFT calculations, 1 exists as a mixture of two enantiomers with an enantiomerization barrier of only 0.5 kcal mol–1. The transition state has Cs symmetry with an endo-CH hydrogen atom. The isomeric iso-1with the Co–H···B bridge is less stable than 1 by 10 kcal mol–1.The formation of 1 and iso-1 from C3B2Me5H and CpCo(C2H4)2 have almost equal activation energies. The isomerization of iso-1 to 1 was shown to proceed as a two-step hydrogen transfer. The bonding of the “extra” hydrogen atom in the related CHB-bridged carborane nido-2,3,5-C3B3R5H2 and metallacarboranes M(C5R5)(C3B2R′5H) (R, R′ = H, Me; M = Co, Rh, Ir) as well as their hydridic isomers MH(C5R5)(C3B2R′5) was compared. According to energy decomposition analysis, the bonding of the parent anion [CpCo(1,3-C3B2H5)]– with metal cations becomes stronger in the following order: K+ < Na+ < Tl+ < Li+ < [RuCp]+. The attractive interactions between the [CpCo(1,3-C3B2H5)]– and Tl+ fragments are 68 % electrostatic and 32 % covalent.
    Berichte der deutschen chemischen Gesellschaft 09/2012; 2012(26). · 2.94 Impact Factor
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    ABSTRACT: The reaction of closo-[B10H10]2− with [PtCl2(PPh3)2] in MeOH at reflux affords the B-methoxy substituted 11-vertex nido-platinaborane compound [(PPh3)2PtB10H10-8-H0.5(OCH3)0.5-10-(OCH3)] (1) and the known species [(PPh3)2PtB10H11-8-(OCH3)] (2) and 1,6-(PPh3)2B10H8 (3). The same reaction under solvothermal condition gives the partially degraded diplatinaborane [(PPh3)2(μ-PPh2)Pt2B9H7-3,9,11-(OMe)3] (4) with a novel nido-Pt2B9H10 skeleton. The new metallaborane compounds have been characterized by spectroscopic methods and single-crystal X-ray analyses. In particular, computational/theoretical chemistry supports the ultimate structural confirmation of 4. The structures of these metallaboranes exhibit interesting intra- and/or intermolecular C–H⋯O hydrogen bonding interactions.
    Polyhedron 01/2012; 31(1):607–613. · 2.05 Impact Factor
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    ABSTRACT: The μ-diborolyl triple-decker complex CpCo(μ-1,3-C3B2Me5)PtMe3 was prepared by reaction of the sandwich anion [CpCo(1,3-C3B2Me5)]–with [PtMe3I]4; according to energy decomposition analysis, attractive interactions between the [CpCo(1,3-C3B2Me5)]– and [PtMe3]+ fragments are ∼64% electrostatic and 36% covalent.
    Mendeleev Communications 01/2012; 22(1):13–14. · 1.05 Impact Factor
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    ABSTRACT: The paramagnetic triple-decker complexes [(Cp*Ru)2{μ-(CMe)3(BCl)(BMe)}] (3b) and [(Cp*Ru)2{μ-(CMe)3(BCl)2}] (3c) are formed by refluxing a mixture of [Cp*RuCl]4 and pentamethyl-2,3-dihydro-1,3-diborole in thf. In CH2Cl2 under air complex 3b slowly looses a hydrogen atom with formation of the diamagnetic triple-decker [(Cp*Ru)2{μ-η5:η6-(CMe)3(BCl)(BCH2)}] (4b) with the unsymmetrical dihydrodiborafulvene derivative in a bridging position. The analogous complex [(Cp*Ru)2{μ-η5:η6-(CMe)3(BMe)-(BCH2)}] (4a) is obtained by interaction of HCl with the triple-decker [(Cp*Ru)2{μ-(CMe)3(BMe)2}] (3a). According to calculations, complexes 4a and 4b have an almost non-distorted triple-decker arrangement with a strong bending of the B=CH2 group toward one of the Ru atoms. For the formation of 3b and 3c the chloro-containing sandwich complexes [Cp*Ru{η5-(CMe)3(BCl)(BR1)}] (2b,c; R1 = Me, Cl) are proposed as intermediates. The constitutions of the complexes are derived from NMR, MS and DFT data, and the molecular structure of 3b is confirmed by an X-ray diffraction analysis.
    Berichte der deutschen chemischen Gesellschaft 05/2010; 2010(19):2911 - 2918. · 2.94 Impact Factor
  • Walter Siebert, Anuradha Gunale
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    ABSTRACT: Over the past years the number of examples of compounds containing a planar-tetracoordinate carbon atom has increased. However, the presence of a carbon atom with a 360° sum of angles does not imply that the species is a derivative of planar methane; there must be an appropriate electronic stabilization. In the case of complexes 21a and 21b the central carbon atom is indeed stabilized by σ-donors and π-acceptors, as required for planar methane.
    ChemInform 01/2010; 31(6).
  • Norbert Weis, Hans Pritzkow, Walter Siebert
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    ABSTRACT: ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.
    ChemInform 01/2010; 30(14).
  • European Journal of Inorganic Chemistry - EUR J INORG CHEM. 01/2010; 2010(19).
  • Andre Weiss, Hans Pritzkow, Walter Siebert
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    ABSTRACT: ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.
    ChemInform 01/2010; 31(16).
  • Walter Siebert
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    ABSTRACT: Pentaalkyl-2,3-dihydro-1,3-diboroles possess unique properties in [LnM(C3B2HR5)] complexes because the neutral heterocycle functions as 4e donor toward metal complex fragments e.g. CpM (M = Co, Rh), (arene)Fe, and others. The specific feature of these complexes is its MeC–H group with methyl in the exo- and hydrogen in an endo-position, forming a bridging 3c,2e C–H–B or an axial M–C–H bonding. This reduces the strength of the C–H bond, and thus complexes of this type exhibit a high reactivity and synthetic potential. Various complexes with a MeC–H group between identical boron centers have been analyzed by X-ray diffraction and NMR studies regarding the bonding of the endo-C–H in bridging or in axial position. The bond lengths of endo-C–H and B–C, the NMR shifts as well as the coupling constant JC–H give information about the bonding situation. In the CpCo(C3B2HMeEt4) sandwich the endo-hydrogen could not be located, its 1H NMR spectrum shows a high-field quartet at −8.8 ppm, and a low JC–H = 81 Hz indicates a weakening of the bond strength. Deprotonation leads to the anion, used as building block for oligo-decker complexes. The complex Ni(C3B2H2Me4)2 having two endo-C–H bonds, exhibits a unique reactivity in loosing hydrogen at ambient temperature and forming 2,3,5-tricarba-hexaboranyl-nickel complexes. Only few complexes allowed to locate the endo-hydrogen in C–H–B position by X-ray diffraction studies, which is supported by calculations. The energy difference between bridging and axial positions is very small.The surprising formation of the slipped 34 VE triple-decker [(Cp∗Ru)2(μ-C6B4H2Me8)] as sideproduct was observed in the reaction of tetrameric (Cp∗RuCl) with C3B2HMe5 and zinc dust to improve the synthesis of the violet sandwich Cp∗Ru(C3B2Me5). The related chloro complex Cp∗Ru(C3B2ClMe4) is a postulated intermediate, however, its transformation into the dinuclear species must include the uptake of two hydrogen atoms, which is not yet clarified. The yellow bis(pentamethylcyclopentadienyl-ruthenium)-μ,η6:η6-hexahydro-tetraboranaphthalene has an unprecedented framework with a bridge-head diborane(4) unit, two additional boron atoms and two MeC–H groups each located between two different boron centers. The endo-hydrogen atoms were not found in the X-ray diffraction analysis, DFT calculations indicate their location in axial positions. 1H NMR data confirm the presence of two endo-H atoms (showing a quartet at – 4.6 ppm), of which only one could be deprotonated by potassium.
    Journal of Organometallic Chemistry 05/2009; 694(11):1718–1722. · 2.00 Impact Factor
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    ABSTRACT: Colliding double deckers: Addition of zinc to a reaction mixture of [{Cp*RuCl}(4)]/pentamethyl-2,3-dihydro-1,3-diborole (C(3)B(2)Me(5)H) in THF leads to three known double- and triple-decker complexes of [C(3)B(2)Me(5)](-), and unexpectedly to the slipped triple-decker (see picture) with two fused diborole rings. The endo C--H bonds of two MeC--H groups donate two additional electrons to achieve the stable 34 VE configuration.
    Angewandte Chemie International Edition 02/2009; 48(8):1429-31. · 11.34 Impact Factor
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    ABSTRACT: The violet ruthenium complex [(η5-C5Me5)Ru(η5-C3B2Me4R1)] (2a, R1=Me) reacts with terminal alkynes R2CCH to give yellow 4-borataborepine compounds [(η5-C5Me5)Ru{η7-(MeC)3(R1B)2(R2C2H)}] (4c, R1=Me, R2=Ph; 4d, R1=Me, R2=SiMe3; 4e, R1=Me, R2=H). The insertion of alkynes into the folded C3B2 heterocycle of 2a causes some steric hindrance, which yields with elimination of the distant boranediyl group the corresponding boratabenzene complexes 5 as byproducts. The analogous reactions with internal alkynes R2CCR2 proceed slowly and afford predominantly the boratabenzene complexes [(η5-C5Me5)Ru{η6-(MeC)3(MeB)(R2C)2}] (5f, R2=Et, 5g, R2=p-tolyl), respectively. In the latter case, three byproducts are formed: methylboronic acid and 1,2,3,4-tetra-p-tolyl-1,3-butadiene (9) due to hydrolysis of the postulated 2,3,4,5-tetra-p-tolyl-1-methylborole (10) and unexpectedly, the cationic triple-decker complex [{(η5-C5Me5)Ru}2{μ,η7-(MeC)3(MeB)2(CH)2}]Cl (11) having two separated CH groups. The new compounds were characterized by NMR, MS, and single-crystal X-ray studies of 4c, 5f, 9 and 11.
    Journal of Organometallic Chemistry - J ORGANOMET CHEM. 01/2009; 694(12):1884-1889.
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    ABSTRACT: Triple-decker complexes with a bridging diborolyl ligand CpCo(μ-1,3-C3B2Me5)M(ring) (M(ring) = RuCp, 4; RuCp*, 5; Co(C4Me4), 6) were synthesized by electrophilic stacking of the sandwich anion [CpCo(1,3-C3B2Me5)]− with the [(ring)M(MeCN)3]+ cations. Structures of 4−6 were confirmed by X-ray diffraction. The electrochemical and spectroelectrochemical behavior of the complexes prepared was studied. DFT calculations of the redox potentials were also performed. Similar bonding properties of anions [CpCo(1,3-C3B2R5)]− and [C5R5]− (R = H, Me) toward [M(ring)]+ cations were established both experimentally (synthesis, electrochemistry, and X-ray diffraction) and theoretically (energy decomposition and Mulliken population analysis).
    Organometallics 01/2009; 28(9):2707-2715. · 4.15 Impact Factor
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    ABSTRACT: Cationic 30 VE triple-decker complexes [Cp*Ru(μ-η7:η7-C5B2Me6H)ML]+ [ML = Co(C4Me4) (3a), RuCp* (4a), Rh(cod) (7a), and Ir(cod) (8a)] with a bridging hexamethyl-4-borataborepine ligand were obtained by electrophilic stacking of the sandwich compound Cp*Ru(η7-C5B2Me6H) (2a) with [ML]+ complex fragments. The reaction of the phenyl-substituted derivative Cp*Ru(η7-7-PhC5B2Me5H) (2b) with [(C4Me4)Co(MeCN)3]+ selectively affords the triple-decker complex [Cp*Ru(μ-η7:η7-7-PhC5B2Me5H)Co(C4Me4)]+ (3b), whereas a similar reaction with [Cp*RuCl]4/TlBF4 produces a 1:3 mixture of cations, the triple-decker [Cp*Ru(μ-η7:η7-7-PhC5B2Me5H)RuCp*]+(4b) and the arene-coordinated [Cp*Ru(μ-η6:η7-7-PhC5B2Me5H)RuCp*]+ (5). Stacking of Cp*Ru(η7-7-PhCH2C5B2Me5H) (2c) with [CpRu(MeCN)3]+ selectively gives the triple-decker complex [Cp*Ru(μ-η7:η7-7-PhCH2C5B2Me5H)RuCp]+ (6c). The dinuclear cations 3–8 were isolated as deep-colored air-stable salts with [BF4]– or [PF6]– anions in moderate to high yields. Structures of 3bPF6, 4aBF4, 7aBF4, and 8aBF4 were confirmed by X-ray diffraction studies. Energy decomposition analysis of complexes CpRu(ring) and [CpRu(ring)RuCp]+ (ring = Cp, C5BH6, C5B2H7) revealed that the insertion of BH units makes the bonding between [ring]– and [RuCp]+ more covalent. According to Mulliken population analysis, weakening of π-donation and strengthening of δ-back donation occur in the same order. The electrostatic character of the bond and the contribution of σ-donation to the covalent bonding are higher in the case of bifacially bonded rings. The boron-containing triple-decker complexes are considerably more stable than the cyclopentadienyl analog. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)
    Berichte der deutschen chemischen Gesellschaft 06/2008; 2008(21):3320 - 3329. · 2.94 Impact Factor
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    ABSTRACT: The syntheses of diboryl porphyrin complexes [(BX2)2(ttp)] (ttp: dianion of tetra-p-tolylporphyrin) and the B-B single-bond diboranyl complexes [(BX)2(ttp)] (X=F, Cl, Br, I) are given. The former are prepared from the reactions of BX3 (X=F, Cl) with [Li2(ttp)] and the latter from B(2)Cl(4) (X=Cl), the reaction of SbF3 with [(BCl)2(ttp)] (for X=F), and, in the cases of X=Br or I, in a remarkable reductive coupling reaction resulting directly from the reaction of BBr3 or BI3 with [Li2(ttp)]. Density functional theory (DFT) calculations on the thermochemical parameters for the reductive coupling reactions (and those calculated for related dipyrromethene complexes) indicate that a combination of the reducing ability of bromide and iodide ions combined with the constrained environment of the porphyrin ligand contribute to the driving force. The reductive coupling is also observed in the reaction of [(BCl2)2(ttp)] with nBuLi to give [(BnBu)2(ttp)], which was characterised crystallographically. The reaction of [(BCl)2(ttp)] with catechol gives a boron catecholato porphyrin complex, [B2(O(2)C(6)H(4))(ttp)]. Chloride abstraction from [(BCl)2(ttp)] gives the planar dication [B2(ttp)]2+, whereas chemical reduction of [(BCl)2(ttp)] by using magnesium anthracenide gives a neutral complex, [B(2)(ttp)], in which the TTP ligand has been reduced by two electrons to give an unusual example of an isophlorin complex. The cationic and neutral complexes [B2(ttp)]2+ and [B2(ttp)] were characterised through a combination of spectroscopic data that is supported by DFT calculations on the porphine analogues.
    Chemistry 02/2007; 13(21):5982-93. · 5.83 Impact Factor
  • Matthias Driess, Walter Siebert
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    ABSTRACT: In the heterocycle (EtC)2(MeB)2S the sulfur atom may be replaced by a CR2 or a NR group; however, attempts to prepare the corresponding phosphorus compound via this route were unsuccessful. Also insertion of phenyl-phosphinidene [C6H5P] into the B-B bond of the 1,2-dihydro-1,2-diborete 2 (R1=N(CHMe2)2) failed. The reaction between the cis-diborylethene 3 (R1=N(CHMe2)) and C6H5PLi2 in C6H6 results in the formation of 1 in 84% yield. The yellowish compound can be sublimed, it is moderately stable in air due to electronic and steric shielding of the boron atoms by the amino groups.
    Phosphorous and Sulfur and the Related Elements. 01/2007; 30(3).
  • Russian Chemical Bulletin 01/2007; 56(5):1090-1092. · 0.42 Impact Factor
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    ABSTRACT: The X-ray diffraction study of the violet ruthenium sandwich derivative 2b revealed a folding along the B⋯B vector of the 1,3-diborolyl heterocycle of 40.7°, which is very similar to that in the green iron complex 1b. The molecular and electronic structure of the 1,3-diborolyl complexes of iron and ruthenium have been studied by density functional theory (DFT) with the B3LYP functional and extended triple-ζ basis sets. It is shown that the folding of the diborolyl ligand in the title complexes is due to electronic factors. The model complexes with a planar diborolyl ligand 1′ (M = Fe) and 2′ (M = Ru) are by 24.9 and 24.5 kcal/mol less stable than the equilibrium folded structures of 1 and 2, respectively. The electronic structures of 1 and 2 show similarities to those of the 18 VE species due to the participation of the σ(B–C2) bonds in the stabilizing diborolyl–metal bonding. The electronic spectra of 1 and 2 have been studied with the time-dependent DFT method. The absorptions observed in the visible range in the electronic spectrum of the title complexes are assigned as spin-allowed d–d transition with an admixture of metal to diborolyl (π∗) charge transfer (MLCT).
    Journal of Organometallic Chemistry 10/2006; 691(21):4565–4572. · 2.00 Impact Factor
  • Walter Siebert, Joseph Edwin, Manfred Bochmann
    Angewandte Chemie 01/2006; 90(11):917 - 918.

Publication Stats

297 Citations
816.75 Total Impact Points

Institutions

  • 1984–2013
    • Universität Heidelberg
      • Institute of Inorganic Chemistry
      Heidelburg, Baden-Württemberg, Germany
  • 1975–2006
    • Philipps-Universität Marburg
      • Fachbereich Chemie
      Marburg an der Lahn, Hesse, Germany
  • 1997–1999
    • Heidelberg University
      Tiffin, Ohio, United States
  • 1977–1985
    • Max Planck Institute for Coal Research
      Mülheim-on-Ruhr, North Rhine-Westphalia, Germany
  • 1980
    • Universität Konstanz
      Constance, Baden-Württemberg, Germany
  • 1969–1977
    • University of Wuerzburg
      • Institute of Inorganic Chemistry
      Würzburg, Bavaria, Germany